1. Field of the Invention
The present invention relates to a signal coder and, more particularly, to a signal coder for high quality coding of wide-band signals such as speech and music at low bit rates.
2. Description of the Related Art
As a system for highly efficiently coding speech signals, CELP (Code Excited Linear Prediction Coding) is well known in the art, as disclosed in, for instance, M. Schroeder and B. Atal, "Code-excited linear prediction: High quality speech at very low bit rates", Proc. ICASSP, pp. 937-940, 1985 (Literature 1), and Kleijn et al, "Improved speech quality and efficient vector quantization in CELP", Proc. ICASSP, pp. 155-158, 1998 (Literature 2).
In these well-known systems, on the transmitting side spectral parameters representing a spectral characteristic of a speech signal are extracted from the speech signal for each frame (of 20 ms, for instance) through LPC (linear prediction). Also, the frame is divided into sub-frames (of 5 ms, for instance), and parameters in an adaptive codebook (i.e., a delay parameter corresponding to the pitch cycle and a gain parameter) are extracted for each sub-frame on the basis of the past speech signals, for making the pitch prediction of the sub-frame noted above with the adaptive codebook. The optimum gain is calculated by selecting an optimum speech codevector from the excitation codebook (i.e., vector quantization codebook) consisting of noise signals of predetermined kinds for the speech signal obtained by the pitch prediction. Thus the excitation signal is quantized. An excitation codevector is selected which minimizes the error power between a synthesized signal from selected noise signals and an excitation signal obtained by the pitch prediction. An index representing the kind of the selected codevector, an index representing a gain codevector, the spectral parameter, a delay parameter corresponding to the pitch cycle and a gain parameter are combined in a multiplexer and then transmitted.
The above prior art systems have a problem that a great computational effort is required for the optimum speech codevector selection. This is attributable to the facts that in the systems disclosed in Literatures 1 and 2 the filtering or convolution is executed for each codevector, and that this computational operation is executed repeatedly a number of times corresponding to the number of codevectors stored in the codebook. For example, with a codebook of B bits and N dimensions, the computational effort required is N.times.K.times.2B.times.8,000/N (K being the filter order or impulse response length in the filtering or convolution). As an example, when B=10, N=40 and K=10, 81,920,000 computations per second are necessary, which is very enormous. This problem is increasingly more serious the more the input signal band is higher than the telephone band and the higher the sampling frequency.
Various systems have been proposed to reduce the computational effort required for the excitation codebook search. For example, an ACELP (Algebraic Code Excited Linear Prediction) has been proposed. For this system, C. Laflamme et al, "16 kbps wideband speech coding technique based on algebraic CELP", Proc. ICASSP, pp. 13-16, 1991 (Literature 3), for instance, may be referred to. In the system shown in Literature 3, an excitation signal is represented by a plurality of pulses, and the position of each pulse for transmission is represented by a predetermined number of bits. The amplitude of each pulse is limited to +1.0 or -1.0, and it is thus possible to greatly reduce the computational effort for the pulse search.
Any of the techniques described above permits obtaining comparatively good sound quality with speech signals. However, with speech signals of a plurality of speakers speaking in a conference or the like or music signals produced by a plurality of different musical instruments and containing a plurality of different pitches, low bit rates result in extreme sound quality deterioration.